Has anyone ever flown Spirit? is it that awful? my Chicago bud who flies back every other week to KC said its never full and super cheap. I looked at their card and it's like 5,000 miles per round trip off peak flight, and they give you 15k sign up and 2x miles for each dollar spent. I only want to travel on the cheap, don't care about upgrades, anyone ever used this card and think it would be worth it?